Optics-based instruments may utilize various types of optical elements for modifying light incident on the optical elements. Depending on the type, an optical element may modify light by, for example, filtering, collimating, focusing, inverting, beam steering, beam splitting, attenuating, etc. As one specific example, optical filters are utilized when it is desired to transmit light at a specific wavelength while preventing transmission of light at other wavelengths. An array of optical filters having different wavelength transmission characteristics may be provided to enable a user of the instrument to select a wavelength for transmission among different wavelengths made available by the array of optical filters. The array of optical filters may be mounted in a filter holding device such as a filter wheel that allows a selected optical filter to be rotated into an operating position at which the selected optical filter becomes part of an optical path and thus is effective for filtering light propagating along that optical path. Another type of filter holding device is a filter slide that allows a selected optical filter to be linearly translated into the operating position. Other types of optical elements, such as lenses, windows, mirrors, etc., may likewise be supported as an array in an optical element holding device.
Examples of instruments that often provide optical element holding devices such as filter wheels include various types of microscopes, one example being a confocal microscope that acquires images of a sample under analysis at different wavelengths (colors). Another example are devices (e.g., optical readers, plate readers, multi-mode readers, etc.) that measure a particular type of optical property or activity of a sample such as reflection/scattering, luminescence, fluorescence, absorbance, etc. A further example are devices that produce spectral data from a sample, such as spectroscopy devices, spectrophotometers, etc. Some of these optical instruments provide both an “excitation” light path from a light source to the sample under analysis and an “emission” light path from the sample to a light detector. That is, the excitation light path is utilized to irradiate the sample, and the emission light path is utilized to transmit the light reflected or emitted from the irradiated sample to the light detector for measurement or imaging. Such instruments may include both an optical element holding device (e.g., an “excitation” filter wheel) positioned in the excitation light path and another optical element holding device (e.g., an “emission” filter wheel) positioned in the emission light path.
An optical element holding device typically provides a number of individual mounting locations, referred to herein as optical element receptacles, at which respective optical elements are retained. Individual optical elements may be loaded into respective receptacles and thereafter removed as needed. In an ideal case, the optical element holding device retains each optical element in a fixed, repeatable position in the corresponding receptacle. In this way, when any of the optical elements currently mounted to the optical element holding device is moved into the optical path, that optical element is in the same position relative to the optical path as any other optical element previously or subsequently moved into the optical path. Also, when any of the optical elements currently mounted to the optical element holding device is replaced with a new optical element in the same receptacle, the new optical element will be in the same position in the receptacle as the optical element previously occupying that particular receptacle. Moreover, for many types of optical elements (e.g. filters and certain types of lenses), in the ideal case the optical element holding device retains each optical element in a perfect orientation (or perfectly aligned position) in the corresponding receptacle. When an optical element is retained in the perfectly aligned position and is moved by the optical element holding device into an optical path, the optical filter will be perfectly aligned with the optical path and thus with the light beam propagating along the optical path. Consequently, the light beam follows a straight path (e.g., is not turned or shifted) through the optical element holding device including while the light beam passes through the thickness of the optical element.
However, in practice some optical element holding devices provide slots through which the optical elements are loaded and thereafter removed. For example, a wheel-type holding device may provide radial slots such that the optical elements are loaded and removed along radial directions. Such holding devices provide a small but perceptible clearance in each optical element receptacle to facilitate loading and removal of the optical elements. That is, each receptacle is sized such that there is some open space between the optical element residing in the receptacle and the surrounding structure of the holding device. This clearance may allow the optical element to be inadvertently loaded into the receptacle with an angular tilt, which results in the light beam being shifted or tilted. Additionally, after loading an optical element into the receptacle (even if properly done such that the optical element is initially well aligned), the optical element may become misaligned over time. For example, operational vibration may cause the optical elements to shift or tilt over time. Misalignment may be problematic for various types of optical elements. For example, when several optical filters (i.e., different color filters) are utilized during a single image acquisition, these angular tilts in the optical filters may induce a phenomenon referred to as “pixel shift.” Pixel shift refers to the misalignment between the individual color images that occurs as a result of the optical element tilt. When these misalignments are large they are easily seen in the resultant image. Existing means for affixing optical filters and other types of optical elements require rigid mechanical fastening methods such as threaded collars or retaining features that offer each of replacement. One example is disclosed in U.S. Pat. No. 6,313,960.
Therefore, there is a need for improving retention and alignment of optical elements in filter wheels and other types of optical element holding devices. There is also a need for mitigating or eliminating problems caused by the clearance associated with the optical element receptacles of optical element holding devices, such as misalignment and attendant disadvantages.